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So, we’ve made a strand circle which rings our original point positions from the simulation, now let’s make those circles align with the rotation of the simulated particle. In practice this is very, very simple. Just insert a “rotate vector” node after you calculate the coordinates on a circle for the strand points, and then use the particle orientation as the input rotation.

Ok, that was nice, but why did it work?

Point positions are vectors. Vectors are displacements:

This is one of the simple but critical concepts that is the real purpose for me writing this series of posts, because it’s a foundational way of thinking which lets you come up with solutions to problems every day.

What is a point? It’s a place, within a space. A particle can have all kinds of attributes, like color size and orientation. But a point is just a position, it only has a single value, a vector (x,y,z). In a manner of thinking, a point is a vector. The vector which describes a point is a direction and distance from the origin. It’s a displacement from that point of reference. When you talk about “global” and “local” space you are talking about different frames of reference, different points of origin from which to draw a vector describing points.

So, what we really did when we calculated an array of strand positions on a circle was make an array of vectors. Hence, rotating those vectors is really the same thing as rotating the strand point positions to match the orientation of our original particle. Points are vectors, they are offsets (or displacements) from an origin. And ICE is very, very good at doing stuff with points vectors. You can call these manipulations vector math if you want, but that in itself doesn’t make it beyond your average artist, who like ICE are also very, very good at manipulating points. If you are an artist you already have an intuitive grasp of vectors! You just need to define some terms, so that saying stuff like “rotate a vector” translates to the visual adjustments you do in your head day in and day out.

Seeing stars… and why modulo is so handy:

Ok, cool. We have a bunch of strand rings centered and oriented where our simulated particles were. Let’s make the rings star shapes, as a way of talking about another useful technique in ICE (it’s useful all over in fact), making patterns via the modulo function.

Ok, a digression first – some housekeeping. By now you’ve realized this isn’t one of those step-by-step makes-a-scene tutorial, I’m discussing more and glossing over a lot of the details you may need to actually plug all this together. I really should have provided a sample scene earlier, I don’t want you focusing on plugging nodes together, the whole point of this is the underlying ideas. So here you go. A sample scene with nifty comments and stuff. If all you want is a scene that will make circles and stars, there you go. And it was made with an educational license, even. But if you want the ideas used so you can make all kinds of other stuff, well then dear reader, read on.

The modulo function is just an instruction to divide two numbers and pick out the decimal remainder of that division. If you feed a linear sequence of numbers (like the index of an array: 0,1,2,3… call any of these numbers “n”) into the modulo function, you get a value counting up between 0 and 1. You can use this to identify every “n’th” item in your list. In fact if you crack open ICE’s “every n’th particle compound etc you will basically see exactly this.) If you can do things every “n’th” time, you can make patterns. Think about it. Braiding hair, knitting, drawing a dotted line – making almost any pattern involves counting and every “n’th” count doing something differently. Modulo is how you do that kind of thing in ICE (and elsewhere. Hey, realflow has an ICE like system now. And it works in scripting too. This math stuff pops up everywhere. The big secret is this – it’s just a way of looking at things you probably already do really well.

I’m a visual thinker so when I was first learning about modulo I had to scribble on a napkin, with results something like this:

All a star shape is, is a pattern where we take every other point on our circle and change it’s radius. Now we know how to find every other point from our list (the array) of strand positions, so we just change the radius of the formula we used to make the circle for those points. And you get a star.

Cooooool.

Ok, so just one last part to this tutorial, and a brief one – how to take the single circles we made and, using the earth-shaking power of ICE and the post-simulation region, turn that result into a lot of circles: all different and making little atom things like we see in the example video. And in fact, the example scene here already shows you how, so we’re not even going to do much besides discuss it and crack bad jokes. Cheers. – AM

Ok, so we’ve talked about the post-sim region, and showed an example. Just what did I do in there?

Strand basics:

Strands are one of my favorite features of softimage ICE. They are really cool. While it’s beyond the scope of this tutorial to get into everything there is to making strands, here is some foundation… Strands are essentially a bunch of per-point attributes telling ICE how to draw the resulting strands. These attributes contain information about color, orientation, number of points in the strand and so on. In our example, the most important of these to us is the strandPosition array. It is an array, one per each particle, in which each strand points position is stored. This is what we’re going to manipulate. Make a simple particle simulation in ICE, and then add a post-sim tree. We’re going to work in there…

Create a strand for each particle:

There is a compound in ICE called “create strands,” which we’ll use here. I’m not a big fan of this compound, it gets the job done but if you get into ICE strands much I advise building your own. At a minimum, I suggest opening up the “create strands” compound and looking around, and then make a single, simple change. See the compound in there called “calc strand ratios?” Plug it into a new port of the big “set data” node in there, and name it “self.strandRatio.” This is an array which assigns each strand point a value ranging from 0 to 1 along the length of the strand. Think of it as a replacement for a “u” value of a curve, it gives you an idea where on the resulting curve a point is. Take your resulting modified “create strands” compound, plug it in, and set the number of strand segments to 20 or more. (Note: depending on the size of your display you may need to click on images to see them without cropping.)

Drawing circles:

There are a lot of ways to describe circles mathematically, but for our purposes we are interested in getting cartesian coordinates (x and y values for each point on the circle). Without getting into the math suffice it to say a parametic form of the equation for a circle you were exposed to in school (x² * y² = r²) is as follows:

x= a + r cost

y = b + r sint

Where (a,b) is a center point, r is the circle radius and t is an angle ranging from 0 to 2Π (or 360 degrees).

Remember that “strand ratio” value? It ranges from 0 to 1 on the length of the strand… meaning if you multiply that by 360, you get the angle (t) for each point on the strand. So, to draw a circle with ICE strands on the x/z axis (like a hula hoop) you get this:

And the result (on a single point) looks like this:

Add this to the particle point position and you get this:

Note that I took the entire “calc strand ratios” compound (from inside “create strands”) and used it here, rather than getting the strand ratio directly where we saved it earlier. You can do it either way, and it’s slightly slower this way where the strand ratios are calculated over and over per frame rather than just being looked up from where we saved it…. but it saved some room in these screen shots. ;)

Next steps:

Now all we have to do is move the actual particle at the center to close the circle. Since we’re in a post-sim tree, moving the particle around doesn’t invalidate our original simulation, it just makes the change for rendering – as far as the “simulation” ice operator is concerned the particle hasn’t moved. So, let’s make the point position the same as the last point on the strand. To get that last point, we can “pop” it from the strandPosition array. Pop just takes the last member of an array, so it’s a handy way to get the value we want in this case. So we get the strandPosition array, pop the last value, and use that as our new point position.

Pretty simple, isn’t it? In the next post, we will adjust the circle to match the particle’s orientation, and then we’ll use this whole setup to create a series of concentric rings around each simulated point. Cheers – AM

“What happens in the post simulation tree stays in the post simulation tree”

A quick review: ICE operators are evaluated differently depending on where they reside in the construction history (also known as the operator stack or modifier stack.) When an ICE tree is under the modeling region it is evaluated every frame unless a simulation region exists – if it does, it is evaluated once. An ice operator under simulation is evaluated every frame, and all data is updated every frame. In other words, changes persist and appear in the next frame – if you move a point, in the next frame that change is reflected. And when an ice operator is in a post-simulation region, it is evaluated every frame but changes are discarded. The “lower” regions evaluated first, then each region “above” it in the explorer, like so:

This is very, very useful. You can have an entire simulation going in the simulation region, and then do stuff to it prior to display. For instance, you could cull out all particles which aren’t visible to the camera to reduce cache size and speed evaluation. You can calculate per-particle lighting prior to rendering it, to control lighting entirely within ICE. Or, in the example below, you can move points around without altering the original simulation.

Example: Strand shapes

Here’s an interesting “look” done entirely with strands in ICE. A typical particle simulation has been used as input for an ICE operator in a post-simulation region, which uses the simulation as a basis to draw many circular strands.

… So you can see that for this effect the bulk of the work was done in the post-sim ICE tree. I use the point positions and orientation from the simulation as a center point around which I add new particles with strands. In my next post I’ll show exactly what I did, but the point I’m getting at here is that things don’t have to end with merely a simulation. You can get into some very cool stuff by considering each frame of a simulation (or a cached result) as a starting point. You can deform your entire simulation, rig it to a character, light it, or perform housekeeping tasks like camera frustrum culling. You can even treat the entire simulation as a single unit and scatter it – they sky’s the limit.

A quick note about motion blur:

When I wax rhapsodic about the post simulation tree the most frequent argument I run into is that moving particles around in a post-simulation operator invalidates motion blur calculations. This is true. Motion blur is based on the velocity of a particle, which can be considered a vector from the previous point position to the current. In the post simulation operators, the previous location is unavailable… it’s like a dog’s sense of time, only what exists “now” has any meaning. So, you have to do some extra work if you need motion blur. Basically, you store the previous point position into a user variable which can be read by your post simulation ICE tree and use that information to calculate not only how you wish to move a particle, but how you moved it in the prior frame as well. From this you can calculate a valid velocity to pass to the renderer for motion blurring. That sounds awful, but it’s not really that bad. (Still, it would be handy if the devs gave us an easier workflow than this, for instance could they store and give us access to post-sim point positions and velocity or something?)

Pretty basic. Shows how you can use a user attribute to effectively create “states” of a simulation without relying on the state machine. Also contains a compound which is part of my regular toolkit, which forces particles and strands onto the closest surface. Like I said, basic stuff, but thats what these examples are… here it is in case anyone could use it for learning or whatever.

I’ve realized I have, over time, created literally thousands of fast test scenes and examples for coworkers using softimage ICE. Often for really cool stuff which never gets made. Most, like this one, are very simple. Some are quite complex.

Rather than have them sitting around where they don’t do any good, I’m going to try to post them on this blog whenever I happen to have a spare moment. I don’t have the time to discuss each one in detail in a post, but most are pretty self explanatory and should help give people ideas when they are stuck or just looking to play around. Most will contain compounds I’ve made but never taken the time to clean up and present to the public, being engaged in trivialities like earning a living or recuperating from making a living…

Is this useful? A lot of the people I work with are quite skilled and could easily achieve the same results themselves – I don’t want to try to suggest that I’m some kind of awesome guy for coming up with this stuff, or clog the collective internet airways and search engines with spam. So if you think it’s helpful, drop me a note. Likewise, note me if you’d just as soon basic stuff like this was kept to myself. :)

This first scene I’ll post was a quick example I made for an artist who needed a fast asset of this sort. Call it 3-minute lava. All we’re doing here is using ICE to turbulize a surface in respect to its normals and then using this deformation to drive a color blend on the objects material based on distances from a bounding center (I don’t remember why we needed it that way, it’s pretty primitive so maybe it was just fast. But there it is, as it was tossed from me to another artist.)